Magnetic resonance imaging and spectroscopy are known techniques for diagnosing, monitoring, researching, or otherwise characterizing various types of conditions in humans, animals (e.g., pets or clinical research subjects), cadavers, archaeological subjects, and so forth. In such cases, the subject is typically of substantial value, and the magnetic resonance techniques are advantageous as they are deemed to be unlikely to cause injury or damage to the subject.
While magnetic resonance is indeed a generally safe and non-damaging technique, problems can arise if the subject includes or is associated with an electrically conductive object, component, or other element. In such a case, radio frequency excitation used in magnetic resonance can couple with the electrically conductive element so as to cause heating, local electrical discharge, or other adverse effect that can injure or damage the subject. For example, in interventional magnetic resonance, a catheter or other interventional instrument is inserted into a human or animal subject. If the interventional instrument includes metal wires or other conductive components, these can couple with the radio frequency signal output by the transmit coil during magnetic resonance excitation. Likewise, implanted pacemaker wires, orthopedic implants, sensors, or other electrically conductive foreign objects disposed in or on a human or animal subject can cause problems. In forensic magnetic resonance entailing examination of a cadaver, archaeological artifact, or the like, the makeup of the subject may be unknown prior to examination, and an unexpected electrically conductive component hidden inside of a cadaver, Egyptian mummy or the like can cause substantial damage due to heating or other effects of the radio frequency coupling.
The likelihood of damage is enhanced dramatically if the electrically conductive element has a natural resonance frequency at or near the magnetic resonance frequency. In such cases, coupling of the electrically conductive element with the radio frequency signal used in magnetic resonance excitation is dramatically enhanced by natural resonance. The natural resonance frequency of an element can be affected by a wide range of factors. For example, the natural resonance frequency of an interventional instrument can change in response to how the physician holds the instrument, or in response to contact between the instrument and the subject as the interventional instrument is inserted into and moved within the subject, or so forth. In sum, there are numerous situations in which a known or unknown electrically conductive object can unexpectedly couple with the magnetic resonance excitation, possibly in a naturally resonant fashion, so as to lead to excessive heating or other damage to the subject.